He means electrically not thermally. A fresh opened bottle of mayhems is non conductive and will remain so long enough to make sure that the loop is leak proof if you don't mix it with anything. Given enough time, they all become conductive due to impurities.
I have had my rig on and stress testing for 3+ hours with a small puddle of Mayhems pastel red developing on the back of the GPU PCB due to a radiator leak which became more apparent after the water warmed up. If you use dye mixes, i would suggest leak testing without the dye unless it is stated to be non conductive and then adding it in later once everything is fine and dandy.